1. Field of the Invention
This invention relates to the application of new nitroxides in the controlled free radical polymerization of a thermoplastic resin or resins. More particularly, the invention relates to a controlled free radical polymerization process wherein piperazinone and morpholone based nitroxides as well as their adducts are used to provide styrene and (meth)acrylate homopolymers and block copolymer resin products characterized by high monomer to polymer conversion and possessing narrow polydispersity properties.
2. Description of the Prior Art
Stable Free Radical Polymerization (SFRP) refers to a free radical polymerization in the presence of a stable free radical such as a nitroxide. The nitroxide traps carbon-centered radicals to form adducts. The trapping is reversible but the equilibrium is such that most (99%) polymer chains are dormant and only a very small fraction is dissociated. Thus, the concentration of free radicals is effectively lowered at all times in the polymerization as shown below, wherein P.cndot. represents the growing polymer radical, .cndot.ON represents the nitroxide, and M represents a monomer. ##STR1##
This in turn affects propagation and transfer rates but has an even greater effect on termination, since the latter is second order in radical concentration.
Thus, stable free radical polymerization minimizes chain termination reactions, while improving molecular weight and polymer architecture control. On the other hand, the SFRP technology slows down the polymerization process.
In U.S. Pat. No. 5,412,047, issued May 2, 1995 to Georges, et al., processes for the preparation of homopolymers of (meth)acrylic monomers and copolymers containing (meth)acrylate segments by nitroxide mediated polymerization are shown. The polymerization process comprises heating a mixture of a free radical initiator and an oxo-nitroxide, specifically 4-oxo-TEMPO (4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy), as a stable free radical agent, at least one polymerizable (meth)acrylate monomer compound, and optionally a solvent, to form a homopolymeric (meth)acrylate containing thermoplastic resin or resins with a high monomer to polymer conversion and a narrow polydispersity.
Examples of other stable free radical compounds said to be suitable for use in the '047 Patent include: 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO); 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy free radical; 2,2,5,5-tetramethyl-1-pyrrolidinyloxy; 3-carboxy-2,2,5,5-tetramethyl-1 pyrrolidinyloxy; and ditert-butyl nitroxide. As noted in the '047 Patent, though, surprisingly and unexpectedly, the aforementioned stable free radical compounds, and related derivatives, while said to be quite satisfactory for the purpose of moderating the polymerization of a wide variety of different monomer types and comonomers, are completely ineffective when used in homopolymerizations of (meth)acrylate monomers. That is to say, no homopolymeric product formation could be detected by GPC when a mixture of n-butyl acrylate, a free radical initiator such as benzoyl peroxide or AIBN, and a stable free radical compound of the type listed in the '047 Patent were heated for about 10 hours at about 140.degree. C.
A solution to the problem of forming (meth)acrylate copolymers and copolymers was said to be achieved in the '047 Patent by substituting, for example, the carbonyl containing stable free radical 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy (4-oxo-TEMPO) in place of the aforementioned ineffective stable free radical compounds. However, 4-oxo-TEMPO is not a stable free radical when heated above 100.degree. C. in the presence of carbon radicals. Under these conditions it is known by those skilled in the art that 4-oxo-TEMPO decomposes rapidly to phorone and other non-radical compounds as set out in Volodarsky, L. B. et al., "Synthetic Chemistry of Stable Nitroxides", p. 50 (CRC Press 1993) and Yoshioka, T., Higa Shida, S., Morimura, S., and Murayama, K., 44 Bull. Chem. Soc. Jpn., Volume 44, pp. 2207-2210 (1971).
All nitroxides based on TEMPO and its derivatives and used by the inventors of the '047 Patent, including 4-oxo-TEMPO, are problematic, particularly when used in the synthesis of (meth)acrylate homopolymers, styrene-n-butyl acrylate block copolymers, and n-butyl acrylate-styrene block copolymers, in that the polymers can not be synthesized in a controlled fashion with high yields. Further, these nitroxides are undesirable for various practical reasons--that is, the nitroxides cannot be synthetically modified easily. Nor do the nitroxides have a long shelf-life.
Thus, there remains a need for a controlled free radical polymerization process, which can be used to simply and economically synthesize high yields of (meth)acrylate homopolymers as well as styrene-n-butyl acrylate or n-butylacrylate-styrene block copolymers. In this regard, a need also exists for a stable free radical nitroxide for use in the polymerization process, which can be easily synthesized and stored over long periods of time.